Recent advances have given rise to the dramatic increase of channel bandwidth in wireless networks, for example, an IEEE 802.11 wireless local area network (WLAN). While current wireless network physical layer technologies such as IEEE 802.11a and 802.11g operate at 54 Mbps, new standards that operate at speeds up to 630 Mbps are under investigation. Meanwhile, new video coding standards such as H.264 offer much higher compression efficiency than previous technologies. Moreover, emerging WLAN media access control (MAC) technologies such as IEEE 802.11e allow traffic prioritization, giving delay sensitive video traffic a priority higher than data traffic in accessing network resources so that the quality of service (QoS) for video traffic and data traffic can be simultaneously supported. All the above have made the streaming of high-quality video over a wireless network possible.
Video multicasting over wireless networks enables the distribution of live or pre-recorded programs to many receivers efficiently. An example of such an application is to redistribute TV programs or location specific information in hot spots such as airports, cafes, hotels, shopping malls, and etc. Users can watch their favorite TV programs on mobile devices while browsing the Internet. For enterprise applications, an example is multicasting video classes or university announcements over wireless networks in campus. Other examples include movie previews outside cinemas, replay of the most important scenes in a football stadium etc.
However, for wireless networks, the transmission error rate is usually high due to the factors such as channel fading and interference. For multicast, the IEEE 802.11 link layer does not perform retransmission of lost packets. A data packet/frame is discarded at the receiving media access control (MAC) layer in the event of an error. Hence, the required quality of service (QoS) may not be guaranteed to the users without good channel conditions. Therefore, additional error protection mechanisms are required to provide reliable services for users and allow adaptation to varying user topology and varying channel conditions of multiple users in a multicast service area.
To achieve reliable video transmission in wireless networks, solutions targeted at different network layers have been proposed, including the selection of appropriate physical layer mode, MAC layer retransmission, packet size optimization, etc.
In the prior art, a cross-layer protection strategy for video unicast in WLANs was proposed by jointly adapting MAC retransmission limit, application layer forward error correction (FEC), packetization and scalable video coding. This strategy is not applicable or appropriate for multicast. In the multicast scenario, the channel conditions for different users are heterogeneous, which means the receivers of the same video session may experience different channel conditions at the same time. Adaptation decisions cannot be made based on a single user's feedback. Furthermore, for multicast packets, the IEEE 802.11 WLAN link layer does not perform retransmissions.
In other art, a scheme which combines the progressive source coding with FEC coding was proposed for video multicast over WLANs. That work also addressed the problem at the application layer and jointly considered the source coding parameter and channel coding parameter. However, in that work, there are several drawbacks. First, the fine granularity scalability (FGS) video coder was adopted. In order to achieve fine granularity scalability, video coding efficiency is lost. Second, the scheme in that work did not consider the error resilience of the source coder. Error resilience of the source coder is also an important parameter for robust video multicast services over wireless networks. The new H.264/JVC standard is expected to dominate in upcoming multimedia services, due to the fact that it greatly outperforms the previous video coding standards. Thus, new adaptive joint source and channel coding algorithms are necessary for H.264-based wireless video multicast system.
In video multicast, every user may have different channel conditions and users may join or leave the multicast service during a session so that the user topology can change dynamically. The key issue is, therefore, to design a system to obtain overall optimality for all users or at least as many users as possible. This can be achieved by appropriately allocating available bandwidth at application layer to the source coder and FEC.